Dynamic rhenium dopant boosts ruthenium oxide for durable oxygen evolution

Heteroatom-doping is a practical means to boost RuO2 for acidic oxygen evolution reaction (OER). However, a major drawback is conventional dopants have static electron redistribution. Here, we report that Re dopants in Re0.06Ru0.94O2 undergo a dynamic electron accepting-donating that adaptively boosts activity and stability, which is different from conventional dopants with static dopant electron redistribution. We show Re dopants during OER, (1) accept electrons at the on-site potential to activate Ru site, and (2) donate electrons back at large overpotential and prevent Ru dissolution. We confirm via in situ characterizations and first-principle computation that the dynamic electron-interaction between Re and Ru facilitates the adsorbate evolution mechanism and lowers adsorption energies for oxygen intermediates to boost activity and stability of Re0.06Ru0.94O2. We demonstrate a high mass activity of 500 A gcata.−1 (7811 A gRe-Ru−1) and a high stability number of S-number = 4.0 × 106 noxygen nRu−1 to outperform most electrocatalysts. We conclude that dynamic dopants can be used to boost activity and stability of active sites and therefore guide the design of adaptive electrocatalysts for clean energy conversions.

Results and discussion: 1) Tafel slopes reported for commercial RuO2 deviate from those reported in the literature, which are normally of ca. 60 mV dec-1. (Energy Environ. Sci. 2022,15, 1977-1987 2) Turnover frequency values are stated in h-1 in this manuscript. This should be amended as these are normally reported in s-1 (see JACS Au 2021, 1, 5, 586-597 for representative work in electrocatalysis devoted to TOF estimation) 3) OER long-term testing was performed on carbon paper, which is known not to be the best catalyst support for OER benchmarking due to substrate passivation (ChemSusChem 2017, 10, 4140-4143;J. Mater. Chem. A 2018, 6, 14162-14169;Catal. Today 2017, 295, 32-40). This, along with the catalyst ink/dropcasting/spraycoating quality (J. Power Sources 2017, 353 19-27;Electrochem. Commun. 2017, 85, 1-5) could be artifacts which lead to the low stability of RuO2 and the much prolonged operation of the ReRuO2 catalyst. The authors should discuss these points and their implications.
4) The authors monitor Re and Ru stability with ICP-MS measurements on the liquid samples. It is unclear whether the loss percentages reported refer to the total catalyst loading or if they are normalized based on the relative elemental content. This should be clarified. In addition, the authors should evaluate if and if there is any time-dependent stabilization of the catalyst based on the S-number metric as reported previously (Nat. Commun. 2021, 12, 2231. Finally, Re vacancies should be formed: can these be identified in HAADF-STEM after long-term testing? 5) HAADF-STEM imaging on RuO2 after 20 h testing (Suppl. Fig. 16) still showcases clear lattice domains in the NPs. The authors state a "dissolution-induced lattice collapse", which does not seem to apply here.
6) The very interesting dynamic oxidation state of Re L3-edge XAS study should be backed by a side-byside comparison of the Ru K-edge, which is surprisingly not reported here and should be the mirror image of the results on Re. This point is key as the proposed pathway change to AEM relies on that. 7) Reference 36 refers to an ATR-SEIRAS study performed on ORR. A more suitable reference should be found. In addition, the Raman feature at 1300 cm-1 is fairly sharp in RuO2 but faint in ReRuO2 and should be discussed as it also presents a potential-dependent intensity.
8) The isotope-labelled OER experiments provide interesting information, but it would have made more sense to perform them at the studied OER potentials chosen for XAS so that a clear link between the dynamic role of Re and the reaction pathway would be revealed. If possible, I would recommend the authors to perform them accordingly. 9) Analogous to the cited work Zagalskaya et al. (ACS Catal. 2020, 10, 3650−3657), the manuscript would benefit from DFT calculation which would devote to the role of Ru or Re defects in the OER electrocatalytic pathway as ICP-MS data clearly shows preferential defect formation (see my previous comment) Supplementary information: 1) Fig. 2: Include JPCD diffraction patters of RuO2 and ReO2 should be included for direct comparison 2) Fig. 13: colours used in Re 4f peak deconvolution should be accompanied with a legend related to the specific oxidation state they are ascribed to 3)

Response
We agree with Reviewer #1 that we should evidence there is electron transfer from eg to t2g. O XPS and K edge spectra confirm a charge redistribution instead of electron transfer from eg to t2g.
In response to address this comment directly, we have in our R-MS, p. 8, para. 1, revised the text to read: 'O-related spectra for RuO2 evidence higher average Ru valence states and charge redistribution compared with pristine sample, that is caused by Ru dissolution-induced catalyst degradation (Supplementary Figs. 17c,d) 48,49 .'

Comment 1-3
In page 7, "Further EXAFS fitting showed that the Ru-O peak …(the coordination number N is 1.8)…". Such a low coordination number of 1.8 seems not possible.

Response
The low coordination number of 1.8 corresponds to a sub-shell for Ru-O coordination. Actually, from the Ru K-edge EXAFS fitted results (Supplementary Fig. 18 and Supplementary Table   3), it can be seen that the Ru-O peak can be divided into two distinct sub-shells with interatomic distances of 1.92 Å (coordination number N is 1.8) for Ru-O1 and 2.01 Å (N is 4.1) for Ru-O2, similar to the pristine sample. This Ru-O bond-length distribution corresponds to a total Ru-O coordination number of 5.9 and, therefore, evidences the adoption of distorted Ru-O6 octahedra.
Identical Ru-O bond distance distribution for the RuO6 octahedra is shown in the standard RuO2 crystal with rutile structure.

Comment 1-4
The EXAFS data fittings at Ru K edges ( Supplementary Fig. 18), Re L edge (Supplementary Fig. 19 and Supplementary Fig. 20) do not look fitted well. More careful analysis and re-fitting the EXAFS data should be carried out. The description of the fitting procedure and the approximations used should be given in the Supplemental Materials.

Response
We agree with Reviewer #1. All EXAFS data at Ru K-edge and Re L3-edge under ex-situ and operando conditions were carefully analyzed, re-fitted, and updated. As is seen from Figures R2-4 and Tables R1-2, the quality of the EXAFS fitting has been significantly improved. Figure R2. Ru K-edge EXAFS fitted analyzes for Re0.06Ru0.94O2 prior to and following 50 h stability test. Best-fit parameters are given in Supplementary Table 3.
In response to address directly this comment we have in our:  Tables R1-R2 as Supplementary Tables 3-4 2) R-MS, p. 23, para. 2, included additional text for description of the fitting-procedure and the approximations used in the Methods section as follows; 'XAS analysis was carried out with standard procedures using ATHENA and ARTEMIS modules implemented in IFEFFIT software 57

Ru site dominates and overlaps that for Ru-O-Re site because of the low doping amount of Re in
Re0.06Ru0.94O2, leading to the unchanged Ru K-edge spectra. Therefore, we focused mainly on the Re L3-edge to determine the dynamic behaviour of Re-O-Ru sites.' Figure R6. Operando Ru K-edge EXAFS fitted analyzes for Re0.06Ru0.94O2 at differing applied potential. Measured and computed spectra are in good agreement. Best-fit parameters are given in Supplementary Table 3.
On-site Large overpotential Figure R7. Change in bond length for Ru-O1 and Ru-O2 coordination shells.

Comment 1-6
It is hard to see the difference in the Re L-edge from Figure 3a.

Response
In response to address this, we have, in our R-MS, revised Figure 3a to highlight the change in the Re L3-edge more clearly.  Given the ratio of Re/Ru indeed varied during testing, will the variation of atomic ratio of Re/Ru degrade or promote the electrocatalytic performance?

Response
The Re/Ru ratio was determined via inductively coupled plasma mass spectrometry (ICP-MS).
Because the ICP-MS has a very high accuracy of up to 0.01 ppb, Re/Ru ratio can be precisely determined. The Re atoms were doped in RuO2 lattice via the following: RuCl3 + NaReO4 → RexRu1−xO2 + NaCl Excess NaNO3 ensures 'complete' oxidation of RuCl3 to RuO2. Therefore, the Re doping level is controlled by changing the amount of NaReO4 in the molten salt. Because the only variation during synthesis is the amount of NaReO4, Re0.06Ru0.94O2 and other samples with fixed ratios are repeatable. Additionally, we successfully obtained Re-RuO2 with different doping ratios, Figure   R9. Re doping does not change the rutile structure of RuO2.
In response to address this comment of Reviewer #3 fully, we have in our: 1) R-SI, added Figure R9 as Supplementary Fig. 9 2) R-MS, p.5, para. 1, included the following clarifying text; We have, therefore, in our R-SI: 3) Added Figure R10 as Supplementary Fig. 14 4) p. 9, included additional text as follows; 'As is presented in Supplementary Fig. 14 Figure R10. a, LSV curves for Re-RuO2 with differing Re in O2-saturated 0.1 M HClO4. b, ƞ10 for Re-RuO2 electrocatalyst. c, Tafel plot for Re-RuO2 with differing Re corresponding to a.

Comment 2-2
The author will be appreciated if they can discuss the definition or difference regarding static electron redistribution and dynamic electron accepting-donating in the introduction section;

Response
We agree with Reviewer #3 to discuss the definition of static electron and dynamic electron accepting-donating in the introduction.
In response, we have in our R-MS, p. 3, para. 2, included the following additional discussion text: 'For example, even though conventional heteroatom doping strengthens the lattice oxygen in RuO2 at small overpotential via electronic structural redistribution, the stability is not sufficient for practical application because of demetallation of the modified-Ru site at large overpotentials 36 .
Therefore, dopants that can tune OER performance via dynamic electron distribution under differing potentials are practically attractive.'
In response to directly address this comment of Reviewer #3 we have in our R-MS: 1) p. 5, para. 1, included the following text; 'LSV curves without i-R compensation are presented in Supplementary Fig. 7a Figure R11. LSV curves for catalysts without i-R compensation.

Comment 2-4
The doped Re atoms in Re0.06Ru0.94O2 are highlighted by yellow circle in its corresponding Aberration corrected HAADF-STEM image (Fig. 2b), however, it is confusing why the authors can verify these dots are Re rather than Ru;

Response
In HAADF-STEM heavy atoms are brighter than light atoms. The formation of distinctive bright and dark spots is interpreted as the difference in the degree of localization and inelastic absorption of channeling electrons in individual atoms by analyses of convergent wave fields inside the crystal in both real and reciprocal space (Ultramicroscopy, 2012, 120, 48.).  Fig. 2b can be verified as Re atoms.
Therefore no change has been made to our R-MS to this comment of Reviewer #2.

Comment 2-5
In this work, the activity and stability were evaluated by a different way from the previous paper, i.e., the mass activity and stability number of S-number, the area activity should be supplied as this parameter is more meaningful for performance evaluation, and please detail the description on how to calculate stability number of S-number;

Response
The area activity was computed by normalizing the electrochemical surface area of different catalysts, Figure R12.
The specific area activity for Re0.06Ru0.94O2 outperformed the other samples, demonstrating superior intrinsic OER catalytic activity.
In response to address fully this comment of Reviewer #2, computational details for normalized area activity and S-number have now been included in our R-SI, namely: 1) In our R-SI, p. 29; 'Supplementary Note 4

Specific area activity
The specific area activity was determined by normalizing the ECSA for different catalysts. The specific current density per ECSA (ja) was computed from: where jgeo is the geometric area current density and Ageo the geometric area of the glassy carbon electrode (0.19625 cm −2 ). Cdl was measured from CV in Supplementary Fig. 11

Response
We thank Reviewer #3 for his/her valuable comments and positive recommendation for publication.

Introduction: The authors aimed to summarize very important works in a very short section, and by doing so they fail to cite key literature in the field. See examples as follows:
Reference 8 is not the first report of RuO4 detection: should be substituted by J. Electroanal. Chem.

Response
We agree with this comment of Reviewer #3.
In response to address this directly, we have in our R-MS: 1) Cited this as Reference 8.
2) In R-MS, p. 3, para. 1, included additional text as follows; 'In 1987 researchers reported that rutile-phase ruthenium oxide (RuO2) has significant practical potential to replace IrO2 for acidic OER because of excellent activity 8 .'

Response
In response to address fully this comment, we have in our R-MS: 1) Cited these papers 2) R-MS, p. 3, para. 1, included text as follows;

Response
We agree with this comment of Reviewer #3.
In response to address this comment, we have in our R-MS:  (Energy Environ. Sci. 2022,15, 1977-1987.

Response
The commercial ruthenium (IV) oxide (≥ 99.9 %) was purchased from Sigma-Aldrich without further purification. All LSV curves reported in our MS were determined following 10 CV scans to remove surface-adsorbed contaminations and stabilize the samples. In the article, Energy Environ. Sci. 2022Sci. ,15, 1977Sci. -1987 and, cited references, nanoparticle samples are synthesized via the researcher. In our work, the synthesized RuO2 nanoparticle exhibited a Tafel slope of 50.3 mV dec -1 , which is close to the ca. 60 mV dec -1 . The commercial RuO2 in our work exhibited 76.4 mV dec -1 because of poor stability. Figure R13 presents Tafel slope for C-RuO2 determined from the first scan, as 63.7 mV dec -1 , close to ca. 60 mV dec -1 . However, during the CV scans, the catalyst exhibited decay because of poor stability.  Figure R13. a, LSV curves for C-RuO2 prior to and following 5 CV scans. b, Tafel plot for C-RuO2 prior to and following 10 CV scans.
In response to address this comment of Reviewer #3 directly, we have in our R-MS, p. 21, para.

1, included a revised text, namely:
'To evaluate OER for different catalysts, the working electrode was put through 10 cyclic voltammetry (CV) scans between 1.1 to 1.6, V at a 20 mV s −1 to clean and stabilize the surface of the catalyst.'

Comment 3-6
Turnover frequency values are stated in h -1 in this manuscript. This should be amended as these are normally reported in s -1 (see JACS Au 2021, 1, 5, 586-597 for representative work in electrocatalysis devoted to TOF estimation).

Response
We agree with this comment of Reviewer #3.

Response
We agree with Reviewer #3 that carbon paper is not the best support for OER testing because of passivation at large positive potential. Therefore, we conducted multiple experiments to exclude the impact of carbon paper.
The stability tests for Re0.06Ru0.94O2 and RuO2 were conducted at similar overpotential of < 1.5 V. Based on our results and published findings (e.g. Nat. Catal. 4, 2021, 1012-1023and Nat. Commun. 2022 the carbon paper electrode is suitable for OER testing under these overpotentials. In addition, based on ChemSusChem 2017, 10, 4140-4143 we determined stability of our materials via monitoring Ru dissolution in the electrolyte using ICP-MS, Fig. 1e. The RuO2 has a linear-dependent increased Ru dissolution that evidences a fast decay of catalyst. However, Ru dissolution in Re0.06Ru0.94O2 is slower. We can conclude, therefore, that Re0.06Ru0.94O2 has better OER stability than RuO2. In response to directly address this comment, we have in our R-MS: 1) p. 6, para. 1, included additional explanatory text, namely: 'Importantly, carbon paper is not an ideal support for acidic OER durability testing because of due substrate passivation [42][43][44][45][46]  We also agree with Reviewer #3 that the quality of catalyst ink drop casting/spray-coating influences stability testing. Therefore, we used the same method to prepare the electrode of RuO2 and Re0.06Ru0.94O2 to exclude the impact of coating method on stability testing. Stability measurements were carried out via uniform air-brush spraying of the catalyst on the carbon with a loading mass of 0.1 mgcata cm −2 . The stability test was conducted at temperature of 25 °C. This method is recently reported, e.g. Nat. Commun. 2022, 13, 4871 andJ. Am. Chem. Soc. 2021, 143, 6482-6490. Therefore additionally, we have in our R-MS: 2) p. 21, para. 1, included additional detailed discussion in Method as follows; '  (Nat. Commun. 2021, 12, 2231. Finally, Re vacancies should be formed: can these be identified in HAADF-STEM after long-term testing?

Response
The loss percentage for different catalysts is normalized based on the relative elemental content of Ru or Re. For example for RuO2, the loading mass is 0.2 mg (0.2 mg cm −2 × 1 cm −2 ). The Ru loading mass is 0.152 mg. The dissolved Ru concentration in the electrolyte is 104 ppb and the electrolyte volume is 40 mL. Therefore, the loss percentage for Ru is (104 ppb × 40 mL)/0.152 mg × 100 % = 2.7 %.
In response to address directly this comment of Reviewer #3 we have in our: 1) R-SI, p. 44, included the following additional text for computational details for ICP-MS data; 'Supplementary Note 5

Loss computation
The loss of catalyst during stability testing was computed from: where Dissolved metal concentration is obtained via ICP-MS and electrolyte volume is 40 mL. Our catalyst exhibited a time-dependent S-number, seen in Supplementary Fig. 15. The Snumber for the catalysts increases with time and is attributed to a slower Ru dissolution rate.
To address this, we have in our; 2) R-MS, p. 7, para. 1, revised the text as follows 'The stability number (S-number) for the catalysts was determined via measuring oxygen produced and dissolved metal ion concentration in the electrolyte (Supplementary Fig. 15

Response
The dissolution of metal species and the instability of the oxygen anion during OER causes the activity decay of RuO2. As is shown in our work and in Chem. Sci., 2015,6, 190-196, RuO2 has much better stability than Ru nanoparticles, even with Ru dissolution. Therefore, it is reasonable that the RuO2 following OER testing retains a clear lattice, in accordance with the data in Chem. Sci., 2015,6, 190-196. In response to address this comment, we want to improve our description of 'dissolutioninduced lattice collapse' and make it more accurate and have in our R-MS, p. 3, para. 1, included the following text: 'Ru active sites are damaged in two ways, 1) formation of lattice-oxygen vacancies via latticeoxygen-mediated mechanism (LOM) [12][13][14] , which leads to instability of the oxygen anion 15 , and 2) over-oxidation of Ru atoms to soluble RuO4 species at high overpotential that leads to demetallation of the active sites 16 .'

Response
Please see detailed response to Comment 1-5.

Comment 3-11
Reference 36 refers to an ATR-SEIRAS study performed on ORR. A more suitable reference should be found. In addition, the Raman feature at 1300 cm -1 is fairly sharp in RuO2 but faint in ReRuO2 and should be discussed as it also presents a potential-dependent intensity.

Response
We agree with this two-part comment of Reviewer #3.
In response to address this comment, in our R-MS: i) An additional two references regarding *OOH detection during OER are cited, namely, 1) Nat.
It should be noted that *OOH bands shift toward a lower wavelength direction in alkaline solution compared with those in acidic solution because the H atom in *OOH can form a hydrogen bond with O atom in OH -, resulting in *OOH moving to a lower wavenumber direction in IR spectra.
ii) Figure R15 presents the ATR-SEIRAS spectra of the blank, Au-coated Si prism at different potentials. The potential-dependent broad peak ca. 1260-1300 cm -1 is attributed to the oxidation of the Au-coated Si prism. The 'sharper' signal for RuO2 than that for Re0.06Ru0.94O2 is due to the lower *OOH peak intensity of RuO2 during OER. Figure R15. In situ ATR-SEIRAS spectra for blank Au-coated Si prism recorded during multipotential steps.

Comment 3-12
The isotope-labelled OER experiments provide interesting information, but it would have made more sense to perform them at the studied OER potentials chosen for XAS so that a clear link between the dynamic role of Re and the reaction pathway would be revealed. If possible, I would recommend the authors to perform them accordingly.

Response
The isotope-labelled OER experiments cannot be performed under the same conditions as for XAS.
We applied constant current chronopotentiometric method operando XAS characterization, which differs from isotope-labelled online DEMS using cyclic voltammetry.
Additionally, DEMS is suitable for detecting dissolved gases and not evolved gases. When overpotential reaches 1.6 V (large overpotential), significant evolved O2 bubbles generate on the electrode surface, leading to a poor DEMS signal, as shown in Figure R16. Therefore, the Therefore, no change has been made in our R-MS in response to this comment. Figure R16. DEMS signals for 32 O2 from reaction products for Re0.06Ru0.94O2 in H2 16 O aqueous sulphuric acid electrolyte with CV from 1.1 to 1.6 V.

Comment 3-13
Analogous to the cited work Zagalskaya et al. (ACS Catal. 2020, 10, 3650−3657), the manuscript would benefit from DFT calculation which would devote to the role of Ru or Re defects in the OER electrocatalytic pathway as ICP-MS data clearly shows preferential defect formation (see my previous comment).

Response
Qualitative analyses of the OER mechanism for RuO2 and Re-RuO2 with metal vacancies were investigated via DFT computation. Importantly, the structure for Re-RuO2 with a Re vacancy is the same as for RuO2 with a Ru vacancy following stabilization. We considered Re (vac-RuO2) and Ru (vac-Re-RuO2) vacancies on Re-RuO2 to determine the impact on electrocatalytic performance. As shown in Figure R17, the sample with metal vacancies exhibits a reaction energy of 0.98 eV (vac-Re-RuO2) and 1.01 eV (vac-RuO2) on AEM pathway, respectively greater than that of 0.79 eV for Re-RuO2. Additionally, we compared the minimum required energy for AEM, LOM and OPM on the defects, Figure R18. The defect sample exhibits optimized OPM Mass signal (a.u.)

Time
Evolved bubbles thermodynamic energy, less than that for AEM on Re-RuO2. Therefore, the advantage of Re-RuO2 over pure RuO2 or defect is demonstrated. In response to address this comment of Reviewer #3 directly, we have in our: 1) R-SI, added Figs. R17 and R18, respectively, as Supplementary Information Figs. 36 and 37 2) R-MS, p. 18, para. 1, included discussion text as follows; 'Qualitative analyses of OER mechanism for RuO2 and Re-RuO2 with metal vacancies were assessed via DFT computation. Importantly, the structure for Re-RuO2 with a Re vacancy is the same as for RuO2 with a Ru vacancy following stabilization. We considered Re (vac-RuO2) and Ru (vac-Re-RuO2) vacancies therefore on Re-RuO2 to determine the impact on electrocatalytic performance. As is seen in Supplementary Figs. 36 and 37

Response
We agree with Reviewer #3. In response to address this, we have in our R-SI included the JPCD diffraction patterns for RuO2 and ReO2 in Supplementary Fig. 2. Figure R19. XRD pattern for Re0.06Ru0.94O2 and RuO2. Both these samples exhibit a rutile phase.
No ReOx peak was apparent.

Response
In response to address this comment, we have in our R-SI revised Supplementary Fig. 13. Figure R20. Re 4f XPS spectra for Re-RuO2 with differing Re.

Response
In response to address this comment in our R-SI, the Re-Ru mass activity is defined in